This invention relates to dust in granular material, and more particularly to apparatus and methods for measuring and for reducing dust in granular polysilicon used to grow semiconductor crystal and solar-grade crystal.
Granular polysilicon, such as CVD grown fluidized bed granular polysilicon, is typically delivered to a crystal growing facility in a shipping container. A conventional container has 300 kg of granular polysilicon. The granular polysilicon is typically sized between 400 and 1400 microns, and any particles sized less than 10 microns is considered dust. As a practical matter, all containers include some amount of dust therein.
The prior art has failed to recognize the extent to which dust can affect the yield of high quality semiconductor crystal. Substantial quantities of dust mixed with the granular polysilicon increases the risk of undesirable defects, such as “Loss of Zero Dislocation” (LZD) in high quality semiconductor crystal. While relatively small batches of prior art granular polysilicon have included acceptably low amounts of dust, there has been no reliable system for obtaining such low-dust polysilicon in large quantities using modern continuous manufacturing methods. Accordingly, improved methods of measuring the dust, reducing the dust and specifying maximum allowable dust in the granular polysilicon are needed.